U.S. patent number 4,378,205 [Application Number 06/138,759] was granted by the patent office on 1983-03-29 for oxygen aspirator burner and process for firing a furnace.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to John E. Anderson.
United States Patent |
4,378,205 |
Anderson |
March 29, 1983 |
**Please see images for:
( Certificate of Correction ) ** |
Oxygen aspirator burner and process for firing a furnace
Abstract
Process and apparatus for firing a furnace using oxygen or
oxygen-enriched air as the oxidant gas, comprising injection into
the furnace of a plurality of oxidant jets, through nozzles, in a
spaced relationship to a fuel jet, at a velocity sufficient to
cause aspiration of furnace gases into the oxidant jets before the
latter mix with the fuel jet, in amounts sufficient to lower flame
temperature.
Inventors: |
Anderson; John E. (Katonah,
NY) |
Assignee: |
Union Carbide Corporation
(Danbury, CT)
|
Family
ID: |
22483512 |
Appl.
No.: |
06/138,759 |
Filed: |
April 10, 1980 |
Current U.S.
Class: |
431/5; 431/187;
431/9 |
Current CPC
Class: |
F23D
14/22 (20130101); F23L 7/007 (20130101); C03B
5/2353 (20130101); F23C 9/006 (20130101); F23C
2202/40 (20130101); Y02E 20/34 (20130101); Y02P
40/50 (20151101) |
Current International
Class: |
F23C
9/00 (20060101); F23D 14/22 (20060101); F23D
14/00 (20060101); F23L 7/00 (20060101); C03B
5/00 (20060101); C03B 5/235 (20060101); F23D
013/20 () |
Field of
Search: |
;431/10,8,5,9,12,175,178,187,188,284,351
;239/8,419.3,419.5,422,424.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
2303280 |
|
Jul 1974 |
|
DE |
|
1215925 |
|
Dec 1970 |
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GB |
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2019553 |
|
Oct 1979 |
|
GB |
|
Primary Examiner: Scott; Samuel
Assistant Examiner: Green; Randall L.
Attorney, Agent or Firm: Gogoris; Adda C. Kastriner;
Lawrence G. Ktorides; Stanley
Claims
What is claimed is:
1. A process for firing a furnace by combusting fuel and an oxidant
to produce furnace gases such that a flame temperature lower than
the normal flame temperature is achieved, comprising the steps
of:
(a) providing a furnace zone substantially closed to the
atmosphere;
(b) injecting into said furnace zone at least one jet of oxidant
gas through an orifice, said oxidant gas being selected from the
group consisting of oxygen and oxygen-enriched air having an oxygen
content of at least 30 percent by volume, said jet having a
diameter D at its point of injection and a velocity at said point
of oxidant jet injection at least equal to that given by the
formula:
where V is the velocity of the oxidant jet in ft/sec, and P is the
oxygen content of the oxidant gas in volume percent;
(c) simultaneously with step (b), injecting at least one fuel jet
through an orifice into said furnace zone, the orifice of said fuel
jet being located in substantially the same plane as the orifice of
said oxidant jet, and said plane being perpendicular to the
direction of at least one of said jets, the orifice of said fuel
jet being separated from the orifice of said oxidant jet as
measured in said plane by a distance X, said distance X being
measured from the outer edge of said oxidant jet orifice to the
outer edge of said fuel jet orifice and being at least equal to
that given by the formula:
(d) aspirating furnace gases from the vicinity of said oxidant jet
into said oxidant jet; and
(e) after said aspiration has taken place, mixing said oxidant jet
with said fuel jet, thereby causing combustion to take place.
2. The process of claim 1, wherein the oxygen jet velocity ranges
between about 450-1000 ft/sec.
3. The process of claim 1, wherein a plurality of oxidant jets are
injected through nozzles spaced about the fuel jet.
4. The process of claim 1, wherein during step (d) the amount of
furnace gases aspirated is sufficient to achieve a flame
temperature during subsequent combustion lower than the normal
flame temperature by an amount .DELTA.T at least equal to that
given by the formula:
where .DELTA.T is in .degree.F. and P is the oxygen content of the
oxidant in volume percent.
5. The process of claims 1, 2, 3 or 4 wherein about 5-10% of the
oxidant is directed adjacent to said fuel jet to form an oxidant
envelope, thereby creating a flame front and stabilizing the flame.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process and apparatus for firing an
industrial furnace of the type in which at least the combustion
zone is either not open to the atmosphere or substantially
insulated therefrom, e.g. by a pressure difference, and which is
commonly used for heating materials such as metals (e.g., a bar
reheat furnace, a soaking pit, or an aluminum melting furnace),
glass, etc. More particularly, this invention relates to a furnace
firing method and apparatus which utilize oxygen or oxygen-enriched
air as the oxidant gas instead of air.
It is common practice for air to be employed as the oxidant gas in
industrial furnaces of the type described above. It is also known
that oxygen enrichment of the oxidant gas for combustion, by
substitution of oxygen in place of part or all of the air, can
reduce the fuel requirements for and help increase the production
rate of industrial furnaces. As oxygen replaces air for combustion,
the nitrogen portion is correspondingly reduced in both the oxidant
and the flue gas, thus reducing the total volume of each, on a
per-unit-of-fuel-burned basis, and increasing the oxygen
concentration of the oxidant-fuel mixture. These changes are, in
turn, responsible for the following principal advantages:
(1) Increase in the maximum achievable firing rate for the burners
of a given furnace, which can be used to augment production rate.
With air as the oxidant, the firing rate may be limited by (a) the
air that can be supplied to the burner through the available ducts
and blowers, (b) the volume of combustion products that can be
handled by the flue, and (c) the firing rate that can be tolerated
by the burner, before combustion instability and incomplete
combustion present problems. With an increase in the amount of
oxygen, the lower oxidant and flue gas volumes overcome the first
two limitations, while the lower oxidant volume and higher oxygen
concentration help overcome the third limitation.
(2) Decrease in fuel consumption. With air as the oxidant, the
sensible heat loss to the flue gas is often substantial due to the
high nitrogen content of air. With oxygen enrichment, the nitrogen
content of the flue gas is reduced and the heat content of the flue
gas is decreased resulting in lower sensible heat losses at
comparable off gas temperatures. The overall fuel savings per unit
of production can be very significant.
(3) Decrease in pollution problems relating to entrainment of
particulates, due to the lower flue gas volume. Gas cleaning of all
pollutants is less costly and more effective with a decreased
volume of flue gas per unit of fuel burned.
The extent of the above benefits increases with the degree of
oxygen enrichment. Therefore, use of substantial oxygen enrichment
as well as use of pure oxygen would be desirable in the art. Such
use, however, has been avoided in the art to date, because it
suffers from the following disadvantages:
(1) High flame temperatures. Flame temperature increases markedly
as the oxygen concentration in the oxidant gas increases. This is
undesirable because it results in (a) unusually high heat transfer
rates in a localized region around the flame which can result in
"hot spots" causing damage to the furnace refractory and/or the
furnace charge, and (b) higher nitrogen oxide (NO.sub.x) emissions,
as the kinetics and equilibria of the NO.sub.x formation reactions
are significantly favored by high temperatures. Use of pure oxygen
as the oxidant gas does not solve the second problem by limiting
the availability of nitrogen, because sufficient nitrogen is
usually present in the furnace, through air leaks (which are
usually unavoidable, even in closed furnaces, especially in
industrial scale operations) or in the fuel, to form nitrogen
oxides in environmentally unacceptable quantities, i.e. in amounts
exceeding the acceptable NO.sub.x emission standards.
(2) Low gas momentum in the furnace. The reduction in mass in both
the oxidant and in the fuel, can result in a substantial reduction
in the incoming oxidant gas and fuel jet momentum, which, in turn,
reduces the amount of mixing and recirculation of the gases within
the furnace. Good mixing and gas circulation in the furnace are
necessary to obtain effective heat transfer and uniform heating of
the charge as well as further to avoid localized hot spots.
Thus, although the aforedescribed advantages of using oxygen or
oxygen-enriched air in place of air in industrial furnaces were
known, such use was avoided because it was accompanied by the
aforedescribed disadvantages. There exists, therefore, a need in
the art for a process and apparatus for firing a furnace which
permits use of oxygen or oxygen-enriched air as the oxidant gas,
thereby taking advantage of the benefits such use affords, but
which overcomes the disadvantages set forth above.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to improve the overall
performance and efficiency of industrial furnaces by (a) increasing
the maximum furnace firing rate through increasing the rate of
oxidant introduction into the furnace, (b) decreasing the furnace
fuel requirements by decreasing sensible heat losses to the flue
through elimination of at least a portion of the nitrogen, and (c)
facilitate abatement of pollutants by decreasing the volume of the
flue gas.
It is also an object of this invention to achieve the above
objective through the use of oxygen or oxygen-enriched air in place
of air as the oxidant gas.
It is a further object of this invention to improve the overall
performance and efficiency of industrial furnaces through the use
of oxygen or oxygen-enrichment, while at the same time avoiding the
disadvantages of high flame temperature and low gas momentum
resulting in high NO.sub.x emissions and a non-uniform furnace
temperature distribution, respectively, which disadvantages
normally accompany use of such oxygen or oxygen-enriched air as the
oxidant gas.
It is another object of this invention to improve flame stability
during combustion in an industrial furnace.
It is yet another object of the present invention to provide burner
apparatus for carrying out the foregoing objects.
These and other objects of this invention will become apparent in
light of the following description and accompanying drawings.
SUMMARY OF THE INVENTION
One aspect of the invention comprises a process for firing a
furnace, comprising:
(a) providing a furnace zone substantially closed to the
atmosphere;
(b) injecting into said furnace zone at least one jet of oxidant
gas selected from the group consisting of oxygen-enriched air
having an oxygen content of at least 30 percent by volume and
oxygen, said jet having a diameter D at its point of injection, at
a velocity, at said point of oxidant jet injection, sufficient to
achieve such gas recirculation and mixing within said zone as to
permit substantially uniform heating of the furnace charge, said
velocity being at least equal to that given by the formula:
where V is the velocity of the oxidant jet in ft/sec, and P is the
oxygen content of the oxidant gas in volume percent;
(c) simultaneously with step (b), injecting at least one fuel jet
into said furnace zone said fuel jet being separated from said
oxidant jet by a distance X, said distance X being measured from
the outer edge of said oxidant jet to the outer edge of said fuel
jet, at their respective points of injection, and at least equal to
that given by the formula:
(d) causing aspiration of furnace gases from the vicinity of said
oxidant jet into said oxidant jet, in an amount sufficient to
achieve a flame temperature during subsequent combustion lower than
the normal flame temperature; and
(e) after said aspiration has taken place, mixing said oxidant jet
with said fuel jet thereby causing a combustion reaction to take
place.
A second aspect of the invention comprises burner apparatus
(hereinafter referred to as the "oxygen aspirator burner") for use
with oxygen or oxygen-enriched air as the oxidant gas in firing a
furnace, comprising in combination:
(a) at least one oxidant gas nozzle of diameter D for injecting a
jet of oxidant gas into said furnace chamber, said diameter being
less than that given by the formula: ##EQU1## where D is in inches,
P is the percent oxygen content of the oxidant gas by volume, F is
the burner firing rate in million BTU per hour (MMBTU/hr) and N is
the number of oxidant nozzles; and
(b) at least one fuel nozzle for injecting at least one fuel jet
into the furnace chamber, said fuel nozzle being spaced from the
oxidant nozzle most proximate to it a distance X, where X is
measured from the edge of said fuel nozzle to the edge of said
oxidant nozzle, and where X has a value at last equal to that given
by the formula X=4D.
By practicing of the process of this invention, substantial fuel
savings and increased production rate may be achieved compared with
furnace firing processes using air, while sufficient gas momentum
is created in the furnace to achieve the amount of mixing and gas
recirculation within the furnace necessary for a substantially
uniform temperature distribution, while at the same time the flame
temperature is lowered so as to keep NO.sub.x emissions at levels
below those acceptable by emission control standards.
The theoretical flame temperature, for a fuel and an oxidant, is
the maximum temperature attainable in a flame (assuming an
adiabatic process and instant and complete mixing of fuel and
oxidant) resulting from the combustion of that fuel with that
oxidant.
The term "normal flame temperature" as used herein shall mean the
flame temperature actually attained in a furnace during the
combustion of a certain fuel and a certain oxidant without
aspiration of furnace gases taking place prior to mixing the fuel
and the oxidant. Normal flame temperature shall be close to
theoretical flame temperature (how close depends on the mixing and
heat transfer conditions prevailing in such furnace). When the
flame temperature in a furnace, wherein the process of this
invention is being practiced, is compared to "normal flame
temperature", such comparison is to be interpreted as being made
with reference to a conventional furnace in a process using
equivalent operating parameters, but no aspiration of furnace gas
prior to mixing the fuel and the oxidant.
BRIEF DESCRIPTION OF THE DRAINWGS
FIG. 1 is a graph depicting the variation of theoretical flame
temperature for natural gas as a function of the oxygen
concentration in the oxidant.
FIG. 2 is a graph depicting the flame temperature achieved in an
oxygen aspirator burner using various degrees of oxygen enrichment
in the oxidant gas for different furnace gas recirculation
ratios.
FIG. 3a is a schematic front view, and FIG. 3b is an axial
cross-sectional representation of an oxygen aspirator burner for
carrying out the process of this invention.
FIG. 4 is a schematic representation of an air burner with swirl
flow and refractory block in axial section.
FIG. 5a is a schematic front view, and FIG. 5b is an axial cross
sectional representation of a conventional concentric ring
burner.
FIG. 6 is a schematic representation of a furnace in which the
process of this invention can be practiced and the apparatus can be
used.
FIG. 7 is a graph comparing the NO.sub.x emission levels of an
oxygen aspirator burner with those of a conventional concentric jet
burner.
DETAILED DESCRIPTION
The description of the invention is given with reference to a
particular embodiment, a method and apparatus for firing a furnace
which furnace has a zone substantially closed to the atmosphere,
such as is commonly used in the steel industry for the heating of a
metal charge, or in other industrial combustion applications, e.g.,
in the glass industry for the heating of a glass charge, etc.
According to the present invention, the fuel and the oxidant jets
are injected into the furnace from separate discharge ports. There
may be only one fuel jet surrounded by a plurality of oxidant jets,
there may be only one oxidant jet surrounded by a plurality of fuel
jets, there may be one oxidant jet and one fuel jet, or there may
be a plurality of both. An especially preferred embodiment contains
one centrally located fuel jet surrounded by a plurality of
circularly arranged oxidant jets (preferably 6 to 8). The distance
(X) measured from the edge of the fuel nozzle to the edge of an
oxidant nozzle, (or, in a different embodiment the distance between
the edge of a fuel jet and the edge of the oxidant jet most
proximately located thereto at their respective points of
discharge) must be at least four times the diameter (D) of the
oxidant jet or jets measured at the nozzle exit (i.e., the inside
diameter of the oxidant nozzle).
The oxidant jets must be injected at a velocity sufficient to
create an aspirating effect around and in the vicinity of each
oxidant jet so that furnace gases, consisting essentially of
combustion products and any non-oxygen portion of the oxidant gas,
can be aspirated into said oxidant jets, directly from such
vicinity, i.e. the space surrounding each oxidant jet (as
distinguished from processes which use separate recirculation ducts
and equipment to recirculate combustion products from a different
part of the furnace, such as the flue). For the invention to be
operable, such aspiration must take place before the oxidant and
fuel jets mix. The amount of furnace gases aspirated up to a point
in the oxidant jet at a distance Y from the oxidant nozzle exit
(see FIG. 3b), is directly proportional to the mass flow rate of
the oxidant jet, and inversely proportional to the diameter of the
oxidant nozzle.
Distance X, as previously defined, must be at least 4 times the
oxidant jet diameter at the nozzle and preferably at least 8 times
the oxidant jet diameter. Tests run with distances X equalling up
to 20 such diameters yielded satisfactory results. It has also been
experimentally determined that, in general, at higher average
furnace temperatures and at higher firing rates, a greater spacing
between the oxidant and the fuel jet nozzles may be desirable in
order, for example to keep NO.sub.x emissions below a certain
level, as will be explained below.
The exit velocity of the oxidant jet must be sufficiently high to
create the requisite aspirating effect. However, there is another
factor which affects velocity and which is controlling in the
present invention. As will be explained below, the velocity of the
oxidant jet must be sufficiently high to create sufficient jet
momentum at the exit of the oxidant jet. It is desirable that said
oxidant jet momentum be at least comparable to that of an
equivalent air jet in a conventional air burner and firing
process.
As mentioned before, use of oxygen or oxygen-enriched air in place
of air results in less gas mass input into the furnace (therefore
less gas momentum) and higher flame temperatures. According to the
present invention, aspirated furnace gases are required to play, in
an oxygen or in an oxygen-enriched air system, the role previously
played by nitrogen in an air system, namely to supply the mass
necessary for gas mixing and recirculation and, as an inert
component in the fuel combustion reaction, to act as a diluent and
to decrease the flame temperature of fuel combustion. The
substitution of nitrogen by hot furnace gases in the oxidant stream
is to a considerable extent responsible for the fuel savings
achieved by the present invention.
Therefore, the amount of furnace gas which must be aspirated into
an oxidant jet for purposes of this invention depends on (a) the
gas mass necessary for efficient mixing and gas recirculation
within the furnace, assuming that the process of this invention
will operate under mixing and gas recirculation conditions at least
comparable to those of a conventional process using air, and (b)
the flame temperature desired for the furnace so as to minimize
NO.sub.x emissions and to prevent localized overheating.
Good mixing and gas recirculation are very important to accomplish
uniform heating and also to avoid localized overheating and
accompanying damage to the furnace charge, refractory, etc.
Without aspiration of furnace gases in the oxidant jet prior to
mixing with the fuel, the flame temperature in the furnace, at the
point where the oxidant and fuel mix, would be equal to the normal
flame temperature which is close to the theoretical flame
temperature (how close depends on the efficiency of mixing) for the
particular type of fuel and oxygen content of the oxidant (see FIG.
1). Flame temperature increases with increasing oxygen content of
the oxidant.
FIG. 1, depicts the variation of the theoretical flame temperature
for combustion of natural gas as a function of the oxygen content
of the oxidant gas, assuming complete and instant mixing
conditions. As the concentration of oxygen increases, the
theoretical flame temperature increases markedly from 3370.degree.
F. for air to 5030.degree. F. (the adiabatic flame temperature) for
oxygen. Of course, during actual operation of a conventional
process, the temperature of the combustion products in the
resulting combustion jet would be equal to the normal flame
temperature at the point of mixing and would decrease along the
length of the jet away from the burner as hot combustion gases mix
with the cooler gases aspirating into the jet.
In industrial combustion applications it is important to control
flame temperature for two reasons. First, high flame temperature
favors kinetics and equilibria of NO.sub.x formation reactions; and
second, high flame temperature may cause localized overheating with
its accompanying undesirable effects (damage to furnace charge,
furnace refractory, etc.). Aspiration of furnace gas into the
oxygen jet prior to mixing with the fuel lowers the flame
temperature below the normal flame temperature, and if the amount
of furnace gas is sufficient (depending also on mixing conditions
in the combustion zone and on the temperature of the furnace gas
itself), the flame temperature will be sufficiently low so that
neither overheating nor NO.sub.x formation present problems, even
when pure oxygen is used as the oxidant gas. Measurement of the
NO.sub.x emission enables one, theoretically, to estimate flame
temperature.
The measured NO.sub.x levels obtained by use of this invention has
been extremely low. The decrease of NO.sub.x levels may be
attributed primarily to effective flame temperature control and
only collaterally to oxygen enrichment (and therefore nitrogen
depletion) of the oxidant gas. Flame temperature control is
generally necessary because nitrogen is almost always present in a
furnace, either due to air leaks or by being combined in the fuel,
in quantities sufficient to form (within the prevailing furnace
residence time conditions) significant amounts of NO.sub.x in the
absence of flame temperature control.
As the amount of aspirated furnace gas increases in the oxidant jet
of the oxygen aspirator burner prior to its mixing with the fuel
jet the flame temperature decreases. The extent of flame
temperature decrease depends also on the temperature of the furnace
gas, but the flame temperature as a function of the amount of
aspirated furnace gas bears the relationship depicted on FIG. 2 to
the recirculation ratio R, defined as the ratio: ##EQU2##
By practice of the present invention it is desirable to achieve a
flame temperature lower than the normal flame temperature by an
amount of .DELTA.T at least equal to that given by the formula:
.DELTA.T=400+7.6 (P-21); where .DELTA.T is expressed in degrees F.
and P is the oxygent content of the oxidant in volume percent.
When using oxygen or oxygen enrichment the mass of the oxidant jet
is decreased, compared to that of an air system, for two principal
reasons. First, elimination of part or all of the nitrogen mass
because of oxygen enrichment; and second, lowering of the oxygen
requirement for combustion, as the nitrogen which has been
eliminated no longer has to be heated up. Therefore, the velocity
of the oxidant jet must be increased in order for the jet to have
sufficient momentum to achieve good mixing and gas recirculation in
the furnace, which are necessary for uniform heat transfer within
the furnace.
For purpose of this invention, the minimum oxidant gas velocity
(measured at the mouth of the oxidant nozzle) necessary to achieve
good mixing and recirculation should be greater than that given by
the following empirical equation:
where V is the oxidant gas velocity in ft/sec and P is the oxygen
content of the oxidant in volume percent, assuming that mixing and
recirculation achieved by this invention is to be at least as
vigorous as that achieved in an air system.
Typically, the oxidant gas velocity for a conventional air furnace
is of the order of about 50-100 ft/sec. A furnace using 100% oxygen
and maintaining the same momentum as that of an equivalent air
system would operate in an oxidant gas velocity range of about
450-950 ft./sec., assuming a fuel saving of 50%. In general, in
order to achieve a momentum level comparable to or higher from that
obtained in conventional air systems, the gas velocity should be at
least 500 ft/sec and preferably higher than 800 ft/sec. The
preferred velocity range is 450-1000 ft/sec.
Schematically shown in FIG. 3a, is a front view of an embodiment of
the oxygen aspirator burner of this invention incorporating
features for practice of the process of this invention. FIG. 3b is
a schematic representation of an axial view of the same burner.
Burner 1 has a fuel feed 2 and an oxidant feed 3 leading to a
plurality of oxidant nozzles 4 of diameter D. Oxidant nozzles 4 are
evenly spaced about a circle 5 around the fuel nozzle 6 at a
distance X from the edge thereof. It will be appreciated, however,
that neither circular nozzle arrangements nor even spacing thereof
are essential to the practice of this invention. Rather, such an
arrangement and spacing represents a convenient embodiment. There
are embodiments of this invention in which other arrangements, such
as having the oxidant nozzles in parallel series at a distance X
from and framing one or more fuel nozzles, or having an asymmetric
oxidant nozzle arrangement which would render the flame reducing on
one side and oxidizing in the other, etc., may be preferred. What
is essential is that the distance X between a fuel nozzle and the
most proximate oxidant nozzle be at least equal to four times the
oxidant nozzle inside diameter D so that sufficient space is
created between the corresponding jets to ensure aspiration of
sufficient furnace gas into the oxidant jets 8 before the fuel jet
9 and oxidant jets 8 mix.
Preferably, the fuel nozzle 6 has flame stabilizing means
associated therewith. In FIG. 3, fuel nozzle 6 has an annulus 10
around it, which is connected to the main oxidant feed 3, through
duct 7, through which a proportionally small quantity of oxidant is
injected so as to create an oxidant envelope (11) around the fuel
jet thereby creating a continuous flame front and stabilizing the
flame. 5 to 10% of the oxidant is sufficient for the oxygen
envelope. A complete oxidant envelope is not necessary. It is
sufficient to have a small quantity (5 to 10%) of the oxidant
adjacent to the fuel jet so as to create a flame front.
In operation, the oxidant jets 8 and the fuel jet 9 are injected
into the furnace. Because of the distance X between each of nozzles
4 and nozzle 6 a space 12 is created between jets 8 and jet 9
defined by the front of burner 1 at one end and by area 13 where
the fuel and oxidant jets mix and combustion takes place at the
other end. Furnace gases, which in the case where oxygen is used as
the oxidant gas, essentially consist of combustion products
(assuming efficient mixing and gas recirculation) are aspirated
into the high velocity oxidant jets 8 from the vicinity of such
jets including space 12. The oxidant jets 8 then mix with the fuel
jet 9 to form a resultant jet (not shown) at area 13. Recirculating
furnace gas finds its way into the vicinity of jets 8 including
space 12 where it is again aspirated by oxidant jet 8 to
effectively dilute the oxygen thereof. Thus, the process of this
invention is able to use furnace gas as a substitute for nitrogen
to achieve the same as or lower flame temperature than the normal
flame temperature for a conventional system using the same fuel and
the same oxygen content in the oxidant gas but no aspiration, and
to maintain the same or higher mixing, gas recirculation conditions
and temperature distribution uniformity without increasing NO.sub.x
emission; in fact decreasing such emission.
The invention can be further illustrated by one or more of the
Examples which follow:
Calculations and experiments were conducted using natural gas as
the fuel, having the following composition and heating value:
______________________________________ GAS COMPONENT VOL. %
______________________________________ CH.sub.4 96.0 C.sub.2
H.sub.6 1.6 N.sub.2 1.6 O.sub.2 0.3 C.sub.3 H.sub.8 0.3 C.sub.3
H.sub.6 0.1 i-C.sub.4 H.sub.8 0.1 100.0
______________________________________ Heating Value: MMBTU BTU
(lb-mole) (ft.sup.3 at 60.degree. F.)
______________________________________ (Gross) 0.383 1010 (Net)
0.346 910 ______________________________________
However, the invention may be practiced using other gaseous or
liquid fuels, or a dispersion of solid fuel in a fluid medium, such
as for example: methane, propane, diesel oil, as well as synthetic
fuels such as a mixture of H.sub.2 and CO.
The percent excess oxidant has been assumed such that the oxygen
concentration in the flue is 2 volume percent. This is achieved at
111.6% of stoichiometric oxidant when using air and at 103.1% of
such oxidant when using oxygen as the oxidant gas. The oxygen
aspirator burner used was of the type shown schematically in FIGS.
3a and 3b. Oxygen nozzle diameters of 1/16 in., 3/32 in. and 1/8
in. were investigated. Tests were made using a total of six and
eight nozzles equally spaced around a circle with the fuel nozzle
axis at its center. The diameter of this circle was varied from 2
to 5 inches. Provisions were made to enable a portion of the
oxidant to be passed through an annulus around the fuel nozzle to
stabilize the flame. Different combustion parameters were
investigated and compared with conventional practice in an
experimental furnace 61, a sketch of which is shown on FIG. 6,
designed to simulate industrial operation. The furnace was
refractory-lined 62 with a heat sink 63 at the bottom and with
inside dimensions of 4 ft..times.4 ft..times.8 ft. The output of
the burners 64 when operating in the furnace was typically 0.5 to
1.0 MMBTU/hr.
Three different type of burners were used: one incorporating the
present invention described above and two other conventional
burners, designated A and B shown schematically in FIGS. 4 and 5,
respectively. FIG. 4 shows Burner A, a conventional swirl flow
burner mounted on refractory burner block 41 (4 inch diameter, 11
inches length) of furnace wall 42 in a recessed fashion and
incorporating a central fuel feed 43 (7/16 in. in diameter)
surrounded by oxidant nozzle 44 (3 in. in diameter). Oxidant nozzle
44 contained swirl means 45 for imparting a tangential component to
the oxidant flow which in conjunction with burner block 41 served
to stabilize the flame.
Burner B, shown in FIGS. 5a in front view and 5b in axial view,
consisted of concentric jet nozzles, a fuel feed 51 enveloped by an
oxygen feed 52. Center fuel nozzle 53 (0.242 in. inside diameter)
surrounded by annular oxidant nozzle 54 (0.375 in. inside diameter,
0.625 in. outside diameter).
The invention is further illustrated in light of the following
experimental results:
1. Operating Range Stability
Initially, the oxygen aspirator burner was used without the oxygen
annulus surrounding the fuel stream. The burner operated unstably
with the flame front oscillating back and forth between the back
and front of the furnace. This caused the furnace to vibrate each
time the flame front moved from the back to the front of the
furnace towards the burner. By passing a portion of the oxygen
(about 5-10%. of the total oxygen flow) through the annulus around
the fuel feed, a continuous flame front was established near the
burner face at the oxygen envelope--natural gas interface. This
stabilized the combustion within the furnace, eliminating flame
oscillations and furnace vibrations. The only visible flame front
was that for the small flow of oxygen flowing through the annulus
and reacting with a portion of the fuel. There was no visible flame
front for the combustion reactions between the oxygen jets and the
bulk of the natural gas. This is in contrast to conventional
burners that have a well defined, visible flame.
The burner proved stable operating with oxygen jets having
velocities up to 980 ft./sec. Higher velocities may also be
possible. For example, in one set of tests using 560 ft.sup.3 of
natural gas and 1140 ft.sup.3 of oxygen, eight oxygen nozzles of
1/16 inch diameter were used. About 7% of the oxygen was fed to the
annulus to stabilize the flame and the remainder flowed through the
nozzles. For these conditions, the oxygen velocity was calculated
to be about 980 ft/sec at a pressure of 11 psig at the nozzle exit.
The nozzle had a straight bore thereby preventing supersonic
velocities within the nozzle. The oxygen leaving the nozzle would
be expected to expand, attaining velocities in excess of 980
ft./sec. The burner operated stably for the nozzles at circle
diameters of 2, 3.5 and 5 inches. It was found that the burner also
operated stably with low velocity oxygen jets although this is of
less practical interest.
The burner operated stably for a range of turndown conditions from
high to low firing rates, the turndown ratio used being up to 20:1.
In one set of tests simulating conditions in a specific industrial
furnace, the furnace temperature was kept within a narrow range by
operating the burner alternately at very high and very low firing
rates. For example, in one of the tests, the burner operated at the
firing rates given below:
______________________________________ High Firing Rate Low Firing
Rate ______________________________________ ft.sup.3 Natural Gas
1050 50 ft.sup.3 Oxygen to Nozzles 2040 0 ft.sup.3 Oxygen to
Annulus 90 155 ______________________________________
The flow rates alternated between the low and high firing rates via
fast acting solenoid valves. The burner operated stably at both
firing rates with no instability encountered during the change from
high to low rates or vice versa. No limitations were found in the
range of low and high firing rates tested that could be used for
stable burner operation. This means that the burner stable
operating range is wider than that used in the above tests.
2. Comparison with Conventional Swirl Burner
A comparison, based on nitrogen oxide (NO.sub.x) formation, was
made between the new oxygen aspirator burner and a conventional
swirl burner. As shown in FIG. 4, the swirl burner incorporated
tangential flow of the oxidant and a refractory burner block to
stabilize the flame. The mixing of the oxidant and the fuel as well
as the residence time in the refractory tube for this burner were
such that the temperature of the combustion products was believed
close to the theoretical flame temperature. The following NO.sub.x
measurements were made in the flue gas when using the conventional
swirl burner:
______________________________________ Vol. % O.sub.2 Nitrogen
Oxide in Flue In Oxidant Lbs/MMBTU
______________________________________ 21 0.044 30 0.19 90 0.41 100
0.10 ______________________________________
For these tests the temperature of the combustion gases at the flue
was in the range 2100.degree.-2200.degree. F. The firing rate was
adjusted for each test condition to keep the furnace temperature
and heat transfer rate to the sink approximately constant. Because
of the fuel savings obtained when oxygen replaced part or all of
the air for combustion, the firing rate decreased as the oxygen
content in the oxidant increased. The data indicate that the
NO.sub.x increased with increasing oxygen content in the oxidant up
to 90% O.sub.2. This was expected since the flame temperature also
increased favoring both reaction kinetics and equilibria for
forming NO.sub.x. Between 90 and 100% oxygen, the NO.sub.x
formation decreased due to the lower concentration of available
nitrogen. In industrial furnace operation, the NO.sub.x, formation
when using 100% oxygen in a conventional burner would probably be
much higher than that shown in the table, due to air leakage into
the furnace. The NO.sub.x formation obtained at 90% O.sub.2 in the
experimental furnace may be closer to that which would be obtained
with 100% O.sub.2 in an industrial furnace, assuming a conventional
burner is used in both instances.
When the oxygen aspirator burner using 100% oxygen was tested at
comparable furnace conditions (approximately the same furnace
temperature and heat transfer rate to the sink), the measured
NO.sub.x was of the order of 0.001 lbs./MMBTU. For these tests,
eight nozzles were used--1/16 inch diameter--in circle diameters of
2, 3.5, and 5 inches and 1/8 inch diameter nozzles in a circle
diameter of 2 inches. The NO.sub.x formation for all conditions
investigated was substantially below any known NO.sub.x emission
regulations and standards. The resulting flame temperature when the
furnace gases were aspirated into the oxygen jets prior to mixing
with the fuel was apparently below that at which the kinetics for
forming NO.sub.x would be significant.
3. Comparison with Conventional Concentric Jet Burners (Burner
B)
Tests were conducted comparing the new oxygen aspirator burner of
this invention with a conventional burner consisting of concentric
jets of fuel and oxygen. The oxygen aspirator burner had eight
oxygen nozzles each of 3/32 inch diameter in a circle diameter of 2
inches. A sketch of the concentric jet burner is shown in FIGS. 5a
and 5b. The tests were conducted at two conditions normally
favorable for the formation of NO.sub.x : high furnace gas
temperature, and with air leakage into the furnace. For all test
conditions, the firing rate was 815 ft.sup.3 of natural gas
combined with 1670-1695 ft.sup.3 of oxygen. The furnace gas
temperature was in the range of 2800.degree.-2900.degree. F.* The
air leakage into the furnace was controlled from 0 to 300 ft.sup.3
of air. The results for NO.sub.x formation are plotted on the graph
in FIG. 7.
For both burners, the NO.sub.x emissions increased as the air
leakage rate increased. However, at comparable test conditions, the
NO.sub.x formation was almost an order of magnitude lower for the
aspirator burner as compared to the concentric jet burner. The
level of NO.sub.x emissions for the aspirator burner was always
below any known emission standards for NO.sub.x for all test
conditions investigated.
* * * * *